Variation in the oxygen isotope ratio of phloem sap sucrose from castor bean. Evidence in support of the Peclet effect

Theory suggests that the level of enrichment of O-18 above source water in plant organic material (Delta) may provide an integrative indicator of cont rol of water loss. However, there are still gaps in our understanding of th e processes affecting Delta. One such gap is the observed discrepancy betwe en modeled enrichment of water at the sites of evaporation within the leaf and measured enrichment of the leaf water as a whole (Delta(L)). Farquhar a nd Lloyd (1993) suggested that this may be caused by a Peclet effect. It is also unclear whether organic material formed in the leaf reflects enrichme nt of water at the sites of evaporation within the leaf or Delta(L). To inv estigate this question castor bean (Ricinus communis L.) leaves, still atta ched to the plant, were sealed into a controlled-environment gas exchange c hamber and subjected to a step change in leaf-to-air vapor pressure differe nce. Sucrose was collected from a cut on the petiole of the leaf in the cha mber under equilibrium conditions and every hour for 6 h after the change i n leaf-to-air vapor pressure difference. Oxygen isotope composition of sucr ose in the phloem sap (Delta(suc)) reflected modeled Delta(L). A model is p resented describing Delta(suc) at isotopic steady state, and accounts for 9 6% of variation in measured Delta(suc). The data strongly support the Pecle t effect theory.